[Improvement] add synthbet option to preproc_t1

[anroy1]

Describe your changes

Add the synthbet option to preproc_t1

List test packages used by your module

• antsbet.zip
• others.zip

Checklist before requesting a review

• Create the tool:
  • Edit ./modules/nf-scil/<category>/<tool>/main.nf
  • Edit ./modules/nf-scil/<category>/<tool>/meta.yml
• Generate the tests:
  • Edit ./tests/modules/nf-scil/<category>/<tool>/main.nf
  • Edit ./tests/modules/nf-scil/<category>/<tool>/nextflow.config
  • Add test data locally for tests with the fork repository
  • Generate the test infrastructure and md5sum for all outputs
• Ensure the syntax is correct :
  • Check indentation abides with the rest of the library (don't hesitate to correct others !)
  • Lint everything. Ensure your variables have good names.

To Do

[AlexVCaron]

Only two small comments, great work 💯 Once addressed, this should be good to go !